A day that's raining or has some other unpleasant weather and you have to be exposed to it. For an explanation, see acronym formed by the first letters.
PVT Blue Falcon: But Drill Sgt, I don't wanna run. It's a Beautiful Army Day!
Drill Sgt: If it ain't rainin', you ain't trainin'! All right everyone, Pvt Blue Falcon wants us to run backwards today!